The Democrat's office said Watson Coleman took a rapid antigen test and the result came back positive.
Dozens of members of Congress have received the first dose of the Pfizer vaccine, but medical experts say it does not provide instant immunity and is not considered fully effective until the second dose is administered weeks later.
Congress' attending physician said lawmakers who hid together for hours in a closed room to avoid Wednesday's mob may have been exposed to the coronavirus by an infected person. A press release from her office on Monday notes that "a number of members within the space ignored instructions to wear masks".
Democratic president-elect Joe Biden, who has made the COVID-19 pandemic a top priority, told reporters he was appalled that Republican politicians refused to wear masks while hunkered down even when others passed them out.
Watson Coleman is isolating at home and awaiting the results of another test. She said in a statement that she is "experiencing mild cold-like symptoms" and is working from home.
Rep. Lisa Blunt Rochester (D-DE) was shown trying to hand out masks in video obtained by Punchbowl News.
As Insider's Kelsey Vlamis reported last week, "about half" of the lawmakers crowded into a secure location during the siege refused to wear masks. "During this time, individuals may have been exposed to another occupant with coronavirus infection", Dr. Brian P. Monahan wrote.
While the lawmakers took refuge during the Capitol assault, at least hundreds of mostly maskless Trump supporters rampaged through the building, hopped up on lies spread by some congressional Republicans and the president that the 2020 presidential election was "stolen" from the him.
Rep. Chuck Fleischman, R-Tenn., said he tested positive on Sunday but a spokesperson likewise told the Post that he was not in the room in question.
Please, wear a mask and social distance. "Chuck" Fleischmann, R-Tenn., who tested positive on Sunday.